The ovarian hormones, estradiol and progesterone, act in brain to mediate complex behaviors, such as female reproductive behavior in rodents. Understanding how these ovarian hormones act in brain is essential to understanding their role in various mental health disorders such as depression. However, the cellular and molecular mechanisms by which steroid receptors mediate the effects of these hormones in brain are not well understood. Recently, a novel class of proteins has been identified, known as nuclear receptor coactivators, that dramatically enhance the transcriptional activity of steroid receptors. While research has led to a much greater understanding of the molecular mechanisms of these coactivators in steroid receptor action in vitro, very little is known about coactivator function in vivo in brain to regulate hormone-dependent gene expression and behavior. This proposal investigates the function of three important coactivators, Steroid Receptor Coactivator-l (SRC-1), SRC-3 and CREB Binding Protein (CBP), in estrogen receptor (ER) action in brain and the regulation of behavior. Aim 1 will determine if SRC-3, which has recently been shown to be essential for female reproductive physiology, is expressed in steroid receptor containing neurons in brain regions known to regulate reproductive behavior. In support, we have found that SRC-1 and CBP are expressed in steroid sensitive cells in behaviorally-relevant brain areas. Aim 1 will also test the hypothesis that these three coactivators physically interact with neural ER in a hormone-dependent manner. Aim 2 will use antisense oligonucleotides to suppress SRC- 1, SRC-3 and CBP expression to investigate the function of these coactivators in ER-mediated activation of three behaviorally-relevant genes: the progesterone receptor, preproenkephalin and oxytocin receptor genes. Aim 3 will use the same antisense approach to test the hypothesis that these nuclear receptor coactivators are critical for the expression of estradiol-induced female reproductive behavior. Consistent with these hypotheses, our preliminary results indicate a functional role for these coactivators in estrogen-dependent gene expression in brain and hormone-dependent reproductive behavior. These studies will greatly enhance our understanding of how these novel coactivators function with steroid receptors in brain to activate behaviorally-relevant genes and regulate complex behaviors. Finally, these nuclear receptor coactivators have been implicated in human disorders, including a form of mental retardation (Rubinstein-Taybi Syndrome) and hormone-dependent diseases such as breast cancer. Studying how these coactivators function in vivo, and moreover in brain, will greatly increase our limited knowledge of the role of these coactivators in human disorders.